KLowe and Shawn, the only conversion that's practical is from CMC 3 to CMC 5 or vice versa, since those two models (from a certain point onward, which I think occurred before 1980) are built on the same circuit board with only one small difference between the the two circuits. Also, the factory can change the low-frequency rolloff point of a CMC amplifier on request (the default for CMC 3/4/5 is 30 Hz, 6 dB/octave; for the CMC 6 it is 20 Hz, 12 dB/octave), and/or they can change the sensitivity setting of the microphone from standard to +5 dB or vice versa. These are all non-traumatic adjustments which were foreseen in the design of the amplifiers.
The CMC 4 and CMC 6, on the other hand, each have their own distinct circuit boards with several significant differences in circuitry between the two of them, as well as between them and the other two types. So a conversion between either of those two types (4 or 6) and each other, or between either of them and either of the other two (3 or 5), is just not a reasonable proposition, unless you call it reasonable to discard around 85% of the parts.
I think that you'd get better value, and that it would feel better and help other people more, to sell what you have and buy what you need instead.
--best regards